As examples of a pneumatic operated hammer for earth and rock drills, a down-on-the hole hammer is disclosed in European patent EP B1 0 336 010 or U.S. Pat. No. 4,015,670. However, the above patent has demerits in that a known piston of the down-on-the hole hammer is geometrically very complex, its repair and maintenance is very difficult and a mechanical trouble occurs frequently to thereby degrade a working efficiency. Further, when the lower end of the piston strikes an anvil portion of a drill bit, a compressed air supply is interrupted to thereby hinder the operation of the hammer.
As an improvement over such a conventional invention, there is disclosed Korean Patent Laid-Open Publication No. 2001-52919 published on Jun. 25, 2001 and entitled “percussive down-the-hole-rock drilling hammer”. However, this patent also embraces shortcomings in that the supply of pressurized air is performed only through an aperture of a central passageway of a piston and outlet apertures of a feed tube, so that the function of the compressed air for the ascending and descending, i.e., the upward and downward movements of a piston is not carried out efficiently, and since the arithmetic ratio of resistance and density of the component parts is applied to the drilling hammer, the drilling hammer does not have a function capable of preventing back flow of underground water or sludge at the time of the actual drilling work.
Furthermore, since a check valve for selectively blocking pressurized air is readily worn or exceeds fatigue strength due to a frequent striking to thereby suffer a partial crack or damage, leading to the frequent stopping of the drilling work. As a result, the work efficiency is deteriorated and the drilling equipment is not easy to repair and maintain.